This invention relates to brake engine limit switches, and in particular it relates to self-adjusting limit switch assemblies.
It has been found to be desirable to have limit switches associated with brakes in order to indicate when the brake linings require replacement or adjustment and to indicate that the brakes have released properly when brake release is initiated. In large brake systems or brake engines, such as are used for example on mine hoists, it is necessary for safe operation that the brake shoes be kept in adjustment and to replace the brake shoes when they are worn. It is also desirable to provide an indication that the brake has released properly each time and is not dragging. In the past, limit switches have been fixedly mounted to the brake assembly so that a limit switch is operated when the brakes release satisfactorily and a limit switch operates when the brake shoes require adjustment because of wear. Because the limit switches were fixed, they required considerable work to position them accurately when the brake engine was assembled and each time the brakes were adjusted for wear the limit switches were re-positioned.
Referring to FIG. 1, there is shown a portion of a known form of brake engine for a mine hoist having a frame 10 on which are mounted two brake cartridges or brake assembly mechanisms 11. These cartridges 11 carry brake linings which, during braking of a mine hoist, press against one flange or side of a friction wheel (not shown). The frame 10 would normally have mounted on it, to press against the other flange of the friction wheel, an opposing pair of cartridges.
While the invention is being described in connection with use on the brake engine of a mine hoist, it will be apparent it can be used on any large brake engine of the disc type where brake linings are pressed against a flat surface of a moving member.
The brake cartridges 11 have an outer disc member 12 to which is mounted a piston shaft 14. On the inner end of shaft 14 is mounted a brake lining (not shown). The shaft 14 is spring biased inwardly to a brake applying position as is a well known safety feature of a mine hoist brake engine. Air conduits 15 carry air under pressure for moving the disc member 12 with piston shaft 14 outwardly, against the spring bias, to release the brake. There is therefore, a brake released position when disc member 12 has moved outwardly and the brake lining is clear of the braking surface on the friction wheel of the mine hoist, and there is a brake applied position when the air pressure holding disc 12 outwards is reduced to its normal level and the disc 12 and shaft 14 move inwardly to apply the brake.
It is known, that in the brake released position the brake lining should be clear of the braking surface, and also that as the brake linings wear there is a position where the mechanism requires adjustment to keep the travel between the brake released position and the brake applied position within predetermined limits. The use of micro-switches, actuated by disc member 12, can give an indication of proper brake release and of the need for brake adjustment. A typical prior art arrangement is shown in FIG. 2.
Referring to FIG. 2, a portion of movable disc member 12 is shown in an opposed relationship with a fixed disc member 16. A diaphragm 17 between disc members 12 and 16 is fixed to the periphery of disc member 17 to provide an air chamber. Air admitted to the air chamber between the surface of disc member 12 and diaphragm 17 pushes against the opposing face of disc member 16 moving the disc member 12 with shaft 14 (FIG. 1) outwardly. In FIG. 2 the brake released position is shown in broken lines and the extreme brake applied position, that is the brake applied position when the brakes need adjustment, is shown in solid line. A mounting arm 18 is fastened to fixed disc member 16 by bolts 19 and extends past the periphery of disc member 16. An L-shaped bracket 20 is mounted by bolts 19A to arm 18 to project in a direction parallel to the direction of movement of disc member 12 and overlying disc members 12 and 16. Two micro-switches 21 and 22 are mounted to bracket 20 with their actuating plungers 23 and 24 facing one another and spaced apart. An actuator 25 is fastened to disc member 12 and projects beyond disc member 12 to terminate in the space between actuating plungers 22 and 23. When the limit switch arrangement just described is set up and operating, the actuator 25 depresses plunger 23 of microswitch 21 when the disc member 12 moves outwardly to the brake released position. Microswitch 21, when actuated, may close (or open) a circuit providing an indication that the brake has released properly. When the brakes are applied, the actuator 25 moves to a position perhaps midway between plungers 23 and 24 when the brakes are newly adjusted, and as the brake linings wear, the actuator 25 moves farther towards plunger 24 in the brake applied condition. When the brakes need adjustment to restore the proper operating parameters, the disc member 12 has moved to such a position in the brakes applied condition that plunger 24 is depressed and microswitch 22 is actuated to close or open a circuit providing an indication that the brakes require adjustment.
It will be seen that the positioning of the microswitches 21 and 22 is critical. These microswitches 21 and 22 have, in the past, been positioned by placing shims between bracket 20 and arm 18, or alternately between disc member and actuator 25. The travel or distance between the brake released position and the brake applied position where brake adjustment is required, is normally a known design distance. Thus the separation or distance between microswitches 21 and 22 is known. However, when the brake engine is first set up the brake release position is not known. If the limit switch assembly, comprising microswitches 21 and 22, bracket 20 and actuator 25 are mounted before the precise position is ascertained and appropriate shimming done, then either microswitch 21 or 22 will be smashed with the first operating cycle. Also, if the brake mechanism is not adjusted immediately a warning indication is issued, a microswitch can be broken by too long a travel. Similarly, when the brake mechanism is adjusted to compensate for brake lining wear, a re-shimming of the limit switch assembly may be required.
The present invention overcomes these difficulties by providing a limit switch assembly which is self-adjusting.
It is therefore an object of the present invention to provide a self-adjusting limit switch for a brake engine.
It is another object of the invention to provide novel limit switch assembly that reduces or prevents damage of microswitches used in the assembly.